Maximizing brightness of laser diode modules is important for many applications, including pumping of fiber lasers and processing of materials directly with diode radiation. Even with many advances over recent years, fiber-coupled laser diode modules still do not reach their theoretically achievable brightness. This disclosure brings the brightness significantly closer to optimum.
A typical prior-art high-power multi-emitter multimode-fiber-coupled laser diode module 10 is illustrated in FIG. 1A and described, for example, in U.S. Pat. No. 7,764,723 issued Jul. 27, 2010 to Ovtchinnikov et al. (“the '723 patent”). At the most basic level, an array of diode emitters 12 output light beams 14 along a light path. In FIG. 1A, each individual diode emitter 12 in the array is stacked on top of the other. Various optics 16, 18, 20 and 22 collimate and shape the beam 14 of each emitter such that each light beam 14 is concentrated and directed into a single multi-emitter beam 19. The beam 19 is directed on the light path towards a fiber 30. The beam 19 generates a beam spot 36 at the location where the fiber facet 31 of fiber 30 may be located. As some multi-emitter laser diode modules 10 may not be fiber-coupled, the beam spot 36 is the launch point of the module 10. If the modules 10 are not fiber-coupled, then lens 22 is generally omitted. In a fiber-coupled module, as illustrated in FIG. 1A, the beam spot 36 is located at the fiber facet 31. As the fiber tip of fiber 30 is a flat surface in FIG. 1A, the fiber tip of fiber 30 is also the fiber facet 31.
Referring to FIG. 1A in greater detail, each broad-area laser diode emitter 12 emits a non-circular beam 14 in the beam direction. Each beam 14 is broad (about 50 to about 200 microns wide) in its slow-axis and narrow (about 1 to about 2 microns) in its fast-axis. The fast and slow axes are transverse to the direction of propagation of the beam. In FIG. 1A, the fast axis is in the x direction, the slow axis is in the y direction and the beam path is in the z direction. Each beam 14 is collimated and shaped by fast-axis collimator 16 and slow-axis collimator 18 to form a wide, vertically thin collimated beam 15. Multiple beams 15 may be stacked in the fast-axis direction (vertically in the x direction in FIG. 1A) from the plurality of diode emitters 12 by a set of mirrors 20 that are slightly offset in the fast-axis direction.
As a result, the multi-emitter laser diode module 10 launches into objective lens 22 a fast-axis-stacked set of thin, wide beams 19 that together fill a region of the objective lens 22. The beams 19 are then focused by objective lens 22 on a beam spot 36 and may be coupled into an output fiber 30 through a fiber entrance facet 31.
A key benefit of the prior-art diode module 10 illustrated in FIG. 1A is that it can achieve relatively high brightness compared to other multimode-fiber-coupled diodes. Brightness is defined as the output power per output cross-sectional area per output solid angle and expressed as watts per square cm per steradian. The same information can also be conveyed by specifying the power in watts and the beam-parameter products (BPPs) in the two orthogonal dimensions transverse to the beam direction. The BPP is defined as the product of beam radius and half-divergence angle in a given transverse dimension and is expressed in mm-mrad. For a fiber 30 with a standard circular core (such as in FIG. 1B), the beam radii and the divergences in the two transverse dimensions equilibrate quickly, thus making the two BPPs equal. To maximize brightness for a given level of power, one needs to minimize the BPPs.
Most multimode-fiber-coupled diodes have relatively poor brightness, i.e. high BPP, because broad-area diode emitters have very asymmetric BPPs in the two transverse dimensions. In the fast axis, these emitters are diffraction-limited, i.e. single-moded, which for a wavelength in the 0.9-1.0 micron range means a BPP of about 0.3; whereas in the slow axis, these emitters are highly multimode, with a width of typically 100 microns and a half-divergence of typically 0.1 radians, giving a BPP of about 5 in this dimension. In existing multimode fiber-coupled diode modules, the BPP is degraded because the modules must accommodate long tails in the near-field profile. Even though anamorphic optics such as prisms and cylinder lenses are able to alter spot sizes and divergences in one axis and not the other, the unfavorable BPPs do not change in either axis.
As a result, regardless of the use of typical optics, the beam 19 launched into the fiber 30 still generally has very asymmetric BPPs. This means that the beam 19 is either asymmetric in the near field (the spatial distribution of the power at the beam spot 36 or fiber entrance facet 31 in the two dimensions), or asymmetric in the far field (the angular distribution in the two dimensions), or both. In the near field, the launched light will quickly spread out in both dimensions to fill the transverse size of the fiber core 32, losing brightness corresponding to however much the facet 31 was initially under filled by the beam. In the far field, the angular distributions of the light in the two dimensions will rapidly mix and yield a net divergence that is a mean between the divergences in the two initial dimensions but is weighted toward the higher divergence. In a circular-core fiber 30 as illustrated in FIG. 1B, if a beam with initially asymmetric BPPs is launched, regardless of whether the asymmetry is manifested in the near field or far field or both, the resultant equilibrated BPP will necessarily be worse (higher) than the average of the initial BPPs, and there will be a net loss of brightness.
In order to achieve high brightness in a circular-core fiber 30, it is important to have roughly equal BPP's in the two dimensions prior to launching into the fiber 30, with a relatively symmetric beam in the two dimensions in both the near field and the far field; and the near-field spot 36 should be sized to fill the fiber core 32 as well as possible, as shown in FIG. 1B. Diode module designs such as that illustrated in prior-art FIG. 1A, or similar to the '723 patent, achieve this through stacking a number of collimated beams 15 from the multiple emitters 12 in the vertical dimension (corresponding to the fast axis of the diodes 12, and the x-axis in FIG. 1A) with the appropriate height per beam 15 so that the spot on the objective lens 22 is roughly square. As a result, after the beams 19 are focused by the lens 22 onto the beam spot 36 or into output fiber 30, the light would have roughly equal divergence in the horizontal and vertical directions. Also, the magnifications in the two dimensions are chosen so that the beam spot 36 on the fiber facet 31 is roughly square and fills the fiber facet 31 as well as possible. Therefore, the BPPs in the two dimensions are roughly equal, and the amount of brightness that is lost after the light equilibrates in the fiber 30 is minimized. The result is that this type of design has higher brightness than most other fiber-coupled diode designs.
However, in order to achieve the conditions necessary for even higher brightness, the design of these prior art diode modules is highly constrained because the number of emitters and their pitch are linked to the width of the emitters and the slow-axis divergence.
Furthermore, designs similar to the '723 patent still fall well short of the maximum theoretically achievable brightness. The reasons for decreased brightness include that the near-field spot 36 projected onto the fiber facet 31 is still rather poorly matched in both shape and power distribution to the fiber core 32, resulting in an under-filled aperture and subsequent brightness loss. This problem can be illustrated by directing one's attention to what is happening between the objective lens 22 and the fiber facet 31, inclusive, in region B of the prior-art design of FIG. 1A. A typical, non-limiting numerical example will also be used concurrently to further illustrate the problem and the solutions proposed in this disclosure.
Referring now to FIG. 1B, a cross-section of the fiber facet 31 of output fiber 30 of FIG. 1A along the line A is illustrated. A typical circular fiber 30 has a fiber facet 31 comprising a flat surfaced face at the beginning of the core 32 at the end of the optical cable. A fiber tip (not illustrated in FIG. 1A or 1B) may extend upstream of the fiber facet 31. A cladding 34 surrounds all of the fiber 30 except for the tip (if any) and fiber facets at both ends of the fiber 30. A typical circular fiber core 32 is 105 microns in diameter with a 125 micron cladding 34 and a numerical aperture (NA) of 0.22.
To improve brightness, the output fiber 30 should be illuminated with as large a beam spot 36 from the multi-emitter laser diode module 10 as possible in the near field, and thereby as low as possible divergence of light, typically 0.15 radians (half-divergence) or less. This substantially under-fills the NA. Unlike in the near field, light in the far field in a fiber does not rapidly spread to fill the available aperture (0.22 NA in this case), so it is useful to launch with as small a divergence as possible (thus higher brightness) since this divergence will essentially be preserved and higher brightness will be present at the fiber output for the user's application.
FIG. 2A illustrates the beams 19 in the fast axis direction and shows near field intensity distribution 202 at the objective lens 22, near field intensity distribution 204 at the fiber facet 31 and far field intensity distribution 206 at the fiber facet 31. As with FIG. 1A, there is no tip on fiber 30, or alternatively, the fiber facet 31 is also the fiber tip.
Starting at the beam spot 36 which is co-located with the fiber facet 31 in FIG. 2A, the near-field intensity distribution 204 is a Gaussian-like distribution. The distribution is a magnified image of the near field of each laser diode emitter 12. The images of each of the emitters 12 is practically identical, so the final distribution (comprising all beams 15 superimposed over top of each other to form beam 19) is essentially the same as the image from any one emitter 12, barring any irregularities. A circular fiber core 32 having diameter of 105 microns can accommodate an inscribed square beam spot 36 of about 75 microns by about 75 microns. But in the fast-axis direction, because of the Gaussian-like distribution 204, the actual full-width-at-half maximum (FWHM) size 208 of the beam 19 must be much less than 75 microns in order to accommodate the long tails or gradually sloping edges of the distribution. If these tails were not accommodated, a considerable amount of power would be clipped at the fiber facet 31, resulting in poor power throughput as well as excessive heating and damage to the fiber. Typically, optics are chosen that provide an approximately 30 times magnification, so since the fast-axis beam height is typically about 1 micron at the emitter 12, the distribution at the fiber facet 31 or beam spot 36 is about 30 microns FWHM, and it can be calculated that the available core aperture of at least 75 microns in the fast axis direction will then capture about 99% of the power in the beam 19.
Still looking at the beam spot 36 and fiber facet 31 in the fast direction, the far-field intensity distribution 206 is a scaled copy of the near-field distribution 202 entering the objective lens 22 because the lens 22 acts as a Fourier transformer in both axes. Since the near field entering the objective lens 22 is where the multiple collimated beams 15 from the individual diode emitters 12 have been stacked side-by-side vertically to form beam 19, the summed intensity distribution 202 is a “top-hat” distribution with steep edges and a relatively flat top, as shown in FIG. 2A before the objective lens 22. With a fast-axis height 210 of about 3 mm (from vertically stacking the beams 15 from each emitter 12) and an objective lens focal length 212 of about 10 mm in both axes, the resulting far-field half-divergence 214 is about 0.15 radians and is uniformly filled.
Turning now to FIG. 2B, beam 19 in the slow axis direction is illustrated including near field intensity distribution 220 at the objective lens 22, near field intensity distribution 222 at the fiber facet 31 and far field intensity distribution 224 at the fiber facet 31. The 10 mm fast-axis and slow axis focal lengths 212 of the objective lens 22 are the same as in FIG. 2A.
Again starting at the fiber facet 31, in the slow-axis direction, the near-field intensity distribution 222 is an image of the wide axis of the emitters, which by the laterally multimode nature of the emitters tends naturally to be close to a steep edged top-hat distribution. Again, the wide-axis image of each emitter 12 is practically identical barring any irregularities, so the final distribution (comprising all beams 15 superimposed over top of each other to form beam 19) is essentially the same as the image from any one emitter 12. The magnification can be chosen such that the near-field width 226 at the fiber facet 31 is about 75 microns so that it fits into the 105-micron core. The far-field distribution 224 at the fiber facet 31 also corresponds to that of the emitters 12 and is close to Gaussian with gradually sloped edged and long tails. The far-field half-divergence at half maximum 228 is about 0.075 radians for typical diode emitters.
In the slow axis, it can be seen that the steep edged distribution 222 of the beam fills the available 75 micron aperture relatively uniformly and therefore efficiently, whereas in the fast axis, much of the 75 micron aperture is filled with the low-intensity tails and gradually sloped edges of the Gaussian distribution 224 of the beam and therefore the filling is about 50% efficient. Furthermore, even if the 75×75 micron inscribed square area was uniformly filled, this would still only fill 65% of the total aperture of the 105 micron circular fiber core 32 as illustrated in FIG. 1B. As a result, the total fill in this numerical example is about 33% efficient. Immediately after launch into the fiber 30, the light spreads out to fill the full aperture relatively uniformly, resulting in a drop in brightness of a factor of about 3.
A further drawback of the prior art occurs in applications where it is desired to provide an external feedback signal to the diode emitters. Such feedback can be used, for example, to ensure that the laser light generated by module 10 is at a tightly controlled wavelength. Conventional Fabry-Perot cavity diode emitters generate light with a center wavelength that is typically controlled only within several nanometers at best. Applications such as optical pumping and optical wavelength multiplexing, for example, often require sub-nanometer wavelength control. As is well-known in the art, it is possible to wavelength-lock a diode emitter by providing feedback preferentially at a desired wavelength, whereby most or all of the light generated by the laser diode is at the desired wavelength. This locking is typically achieved at some cost in output power, corresponding roughly to the power used in the feedback signal. Various techniques exist for providing this feedback, including using structures such as Bragg gratings etched directly in the emitter chip and using wavelength-selective partially reflective optics such as volume holographic gratings adjacent to one of the emitter facets. Although desirable for reasons including cost, reliability, stability, and wavelength precision, it has to date not been practical to lock the wavelength of the emitters 12 using feedback from a wavelength-selective optic situated downstream of a fiber 30 on a multi-emitter module 10. There are two reasons for this. First, as is well-known in the art, poor brightness performance of an optical assembly causes poor power efficiency in the reverse direction. Using the numbers in the above example, a drop in forward brightness by a factor of 3 implies, for backward-traveling power, a drop in power by 3×. Thus, if, for example, a wavelength-selective reflectivity of about 10% is required for reliable locking of a diode emitter, then a reflectivity of about 30% would be required in a wavelength-selective optic downstream of the fiber, resulting in an unattractively high cost of output power from the system. Second, when light is reflected back in the fiber of prior-art fiber-coupled modules and from there transmitted into individual emitters, it has been observed that the back-reflected power is non-uniformly distributed among the emitters, with the outlying emitters receiving the least amount of feedback. Since all of the emitters typically must be reliably locked, the required reflectivity of the downstream optic will be set by the weakest-locked emitter, and the surplus locking power provided to the other emitters will be wasted. The result is a further increase in the required reflectivity of the downstream optic.
As will be described below, the present disclosure teaches a modified design that improves the brightness that can be achieved at the beam spot 36 and immediately after launch into a fiber 30, if the multi-emitter laser diode module is fiber coupled. This design also enables, in fiber-coupled configurations, more efficient feedback to the emitters using downstream optics than is possible in prior-art designs.